A heat exchanger is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted.
http://en.wikipedia.org/wiki/Heat_exchanger

Also:
A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two liquids. This has a major advantage over a conventional heat exchanger in that the liquids are exposed to a much larger surface area, as the liquids spread out over the plate.
http://en.wikipedia.org/wiki/Plate_heat_exchanger

Approach according to COED means:
come near or nearer to in distance, time, or standard; nearing.

American Heritage also suggests:
- A way or means of reaching something; an access: an approach to the bridge.

An example of approach that seems suitable to this situation reads:
As compared to Shell & Tube Heat Exchanger, PHE can give approach of 1°C whereas Shell & Tube Heat Exchanger gives approach up to 5°C. For the same amount of heat exchange the size of the PHE is less ... .

As with this usage of approach in the context heat exchangers, it seems that what you're looking for is "temperature approach". The temperature approach used in a shell and tube heat exchanger is simply the difference between the outlet temperature of one stream and the inlet temperature of the other stream.

I am not so sure about the technical term used for temperature approach by Persian technicians; but I guess, بیلان is widely used for approach in similar senses.

Other possible equivalences include:

میزان دگرسازی مبدل حرارتی
بیلان دگرساز گرمایشی
توان دگرش دگرساز گرمایشی




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Note added at 2 hrs (2007-01-24 15:22:41 GMT)
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I understand some translators may prefer explicitation; But just imagine you want to put "اختلاف دمای آب سرد ورودی و خروجی مبدل حرارتی" in a chart or illustrative figure, let alone in a formula!

Nevertheless, within a text (as is probably the case with this question) explicitation is quite reasonable.

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Note added at 2 hrs (2007-01-24 15:26:03 GMT)
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As with other terms suggested, I am sure that "دگرساز" and "دگرش" is already in use in technical texts. Even Aryanpur has introduced these terms.

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Note added at 8 hrs (2007-01-24 21:14:20 GMT)
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Dear Mr. Razaghi,
I didn't say that "approach" should be translated as بیلان. Undoubtedly, the original terms for "بیلان آبی" and "بیلان گرمای اتمسفر" are "water balance" and "atmosphere radiation (heat) balance" respectively.
However, compare the deffinition for these terms:

temperature approach:
the difference between the outlet temperature of one stream and the inlet temperature of the other stream.

radiation balance:
the difference between radiation of the Earth's surface and trapped radiation

Water Balance:
the difference between evapotranspiration and water storage in soil or the bedrock

Balance (accounting):
the difference between the debit and credit of an account.

balance:
The difference in magnitude between opposing forces or influences



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Note added at 8 hrs (2007-01-24 21:18:58 GMT)
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از دیدگاه فن واژه شناسی، به این روش انتخاب معادل از طریق تغییر زاویه دید بر پایه قیاس قاموسی می
گویند

هر مبدل حرارتی با دو جریان سیال سرد و گرم سرو کار دارد و گرما را از جریان گرم به جریان سرد منتقل میکند.
temperature approach
طبق تعریف برای مبدل های همسو که در آنها دو جریان سرد و گرم در یک جهت حرکت می کنند برابر اختلاف دمای جریان خروجی گرم و جریان خروجی سرد است و در مورد مبدل های نا همسو که جریان گرم و سرد در خلاف جهت هم حرکت میکنند برابر مینیموم اختلاف دمایجریان گرم و سرد در دو طرف مبدل خواهد بود.یعنی ما دو اختلاف دما در دو سر مبدل داریم که کمترین مقدار از بین این دو مورد نظر است.
این تعریف در بسیاری از منابع و جود داره و به عنوان مثال می توانید به صفحه ی 105 کتابی که آدرسش به عنوان منبع آمده مراجعه کنید.
بنا بر این توضیح ترجمه متن مورد نظر شما بر این اساس که مبدل همسو یا غیر همسو است تغییر میکند مثلا با فرض همسو بودن می توان ترجمه کرد:
کارایی مبدل حرارتی صفحه ای تابع اختلاف دمای دو جریان در خروجی آن است

Plate and frame heat exchengers might be used for different fluids than water. So we shouldn't say water temperature , instead we should say hot and cold stream temperatures.So the translation is:
ekhtelaf damaye jaryanhaye sard va garm e khorooji az mobadel